The present invention relates generally to the production of hot dip galvanized steel strip and more particularly to a method and apparatus for minimizing spangling on such strip.
Hot dip galvanized steel strip is produced by passing a continuous steel strip through a bath of molten zinc or zinc alloy. When the strip is withdrawn from the molten bath, the surface thereof is covered with molten zinc which solidifies as the strip cools. Usually, as the molten zinc solidifies, the surface of the strip takes on the characteristic spangled appearance of a galvanized steel product. A spangled surface is undesirable under certain conditions of use, e.g., when the galvanized surface is to be painted. Attempts have been made to minimize spangling, and some of these previous attempts employed spraying onto the molten exterior surface portion of the strip, finely divided particles of a nucleating agent.
The theory underlying the employment of finely divided particles of nucleating agent is that, when such a particle impinges against the strip's molten exterior surface portion, solidification begins at the site where the particle impinges, and the solidification crystal thus formed grows outwardly from the site of impingement. It is these crystals of solidification which produce the spangled appearance, and the less these crystals are allowed to grow, the smaller the spangles appear. Therefore, when the molten exterior surface portion is impinged by a multitude of fine particles of nucleating agent, so many sites for solidification are formed, in such close proximity to each other, that they have virtually no chance to expand, and the visual appearance thus produced is an absence of spangling. The larger the particle size of the nucleating agent, the larger the spangle. Moreover, very large particles of nucleating agent (e.g., 3-5 mm) must be avoided because very large particles cause the formation of large, substantially round, water marks which are readily visible and create an undesirable surface appearance and texture on the strip.
Among the nucleating agents heretofore employed are water or aqueous solutions in the form of fine spray or fog or wet steam. However, a nucleating agent comprising finely divided particles of water or of an aqueous solution has drawbacks. Atomizing or dividing the nucleating agent into fine particles, employing conventional atomizing devices, produces a range of particle sizes including some large particles and can produce some very large particles which, upon impingement against the molten exterior surface portion of the galvanized steel strip, produce large spangles and/or the above-described undesirable surface appearance. Moreover, finely divided droplets of aqueous solution can coagulate during the journey from the spray nozzle to the surface upon which they impinge. A coagulated droplet impinging against that surface can have the same undesirable effect on surface appearance as a large or very large droplet originating as such at the spray nozzle.
Prior art patents which disclose the employment of finely divided particles of water or aqueous solution or wet steam to minimize spangling on hot dip galvanized steel strip include Cook et al., U.S. Pat. No. 2,094,583 and Mayhew, U.S. Pat. No. 3,148,080.